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This question asks what to do about loosing XMPP messages on mobile devices when they don't have a stable connection, but I don't really see why the packages get lost in the first place.

I remember having read that the stream between the server and the client stays open when the connection is suddenly lost and will only be destroyed once the connection times out. This means that the server sends arriving messages over the stream, even though the disconnected client can't receive those messages anymore.

I was happy with that explanation for some time, but started wondering why core XMPP would be lacking such an important feature. Eventually I noticed that ensuring correct transmission in the XMPP protocol would be redundant, as the underlying TCP should already ensure the proper transmission of the message, but as the various problems that arise from the lost message it seems that this isn't true.

Why isn't TCP enough to ensure that the message is either correctly sent or fails properly so the server knows it has to send the message later?

Community
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Kritzefitz
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1 Answers1

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Why isn't TCP enough to ensure a proper transmission (or proper error handling, so the server knows the message has to be sent again) in this scenario?

Application gives the data that needs to be sent across to its TCP. TCP segments the data as needed and sends them out on established connection. Application passes over the burden of ensuring the packet reaches the other end to TCP. ( This does not mean,application should not have re-transmissions. Application level protocol can define re-send of messages if right response didn't come)

TCP has the mechanism of the Re-transmissions. Every packet sent to peer needs to be acknowledged. Until the acknowledgements come in TCP shall have the packets in its sendQ. Once the acknowledgement for the packets sent is received, they are removed.

If there is packet loss, acknowledgements don't arrive. TCP does the re-transmissions. Eventually gives up.Notifies application which needs to take action. Packet loss can happen beyond TCPs control. Thus TCP provides best-effort reliable service.

Prabhu
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  • Sure TCP can't always successfully transmit the message, but as you said, it should notify the application and, in this case, add the message to the offline message and re-submit when the connection is available again. – Kritzefitz Oct 14 '16 at 17:12
  • When the connection if gone, all its state is also gone. The client and server application should restart things from the start. They should have the intelligence to see what data needs to be resent. Not the TCP – Prabhu Oct 14 '16 at 17:16
  • Sure the server should know that it has to send the messager later, but it doesn't. That's what I'm asking about. – Kritzefitz Oct 14 '16 at 17:28